The present invention relates to an automotive transmission, and more particularly to a transmission of the countershaft type.
Many automatic transmissions of the countershaft type have two parallel shafts, i.e., input and output shafts parallel to each other, a plurality of trains of intermeshing transmission gears disposed between the input and output shafts, and clutching means (such as hydraulic clutches, dog clutches, or the like) disposed parallel to each other in the axial direction for selecting power transmission via any one of the transmission gear trains. Engine output power which has been changed in speed by the transmission gear train selected by the clutching means is transmitted to the output shaft (see Japanese Patent Publication No. 62-34979, for example).
More specifically, an automatic transmission of the countershaft type for use with an engine directed transversely in an engine compartment is shown in FIGS. 6 and 7 of the accompanying drawings. The transmission, generally designated by the reference numeral 70, has a input shaft 72 disposed coaxially with and coupled to an output shaft (flywheel) 61 of an engine 60, and an output shaft 73 and axle shafts 74a, 74b which are disposed parallel to the input shaft 72. The transmission 70 also includes four trains of intermeshing transmission gears 81a, 81b; 82a, 82b; 83a, 83b; 84a, 84b for first through fourth gear positions, one train of transmission gears 85a, 85b, 85c for a reverse gear position, four hydraulic clutches 75, 76, 77, 78, and a single dog clutch 79, all disposed between the input and output shafts 72, 73 and parallel to each other in the axial direction. Any one of the hydraulic clutches 75, 76, 77, 78 and the dog clutch 79 is operated to select one of the transmission gear trains for power transmission and speed changing. The engine output power thus changed in speed is transmitted through output gears 87a, 87b and a differential mechanism 88 to the axle shafts 74a, 74b, from which drive power is transmitted to lefthand and righthand road wheels.
FIG. 7 shows the arrangement of the shaft of the transmission. With the transmission 70 coupled to an output end of the engine 60, the input shaft 72 is disposed coaxially with the crankshaft 61 of the engine 60, and the output shaft 73 and the axle shafts 74a, 74b are positioned substantially laterally of the input shaft 72.
The transmission gear trains and the hydraulic clutches (dog clutches or the like may be employed in place of the hydraulic clutches for mechanically enabling and disabling power transmission) are disposed between the input and output shafts and parallel to each other in the axial direction. Therefore, it is difficult to reduce the axial dimension of the transmission. The axial dimension of the transmission becomes larger as the number of gear positions is increased.
With the transmission being directed transversely in the engine compartment, the engine and the transmission coupled to the rear end thereof are required to be placed in the engine compartment so as to extend in the transverse direction of the automobile. If the axial dimension of the transmission is large, any margin of the installation space available in the engine compartment for the engine and the transmission is small. Where the transmission is directed longitudinally in the engine compartment with the engine and the transmission coupled to the rear end of the engine extending longitudinally in the automobile, the rear end of the transmission projects into the passenger compartment of the automobile to a large degree, resulting in the problem of a reduce passenger compartment space in addition to the above problem of the reduced margin of the installation space in the engine compartment. Moreover, the transmission itself suffers a drawback in that the shafts thereof are longer and have reduced rigidity, increasing noise from the gears on the shafts and reducing gear durability. Furthermore, the rigidity of the transmission case is lowered and hence vibration and noise of the power line are increased.
To avoid the aforesaid shortcomings, there has heretofore been proposed, as disclosed in Japanese Utility Model Publication No. 57-9136, a transmission having one more shaft in addition to the two conventional shafts supporting the transmission gears, and a gear train disposed between the additional shaft and one of the two existing shafts and having a clutch means for selecting an auxiliary gear position. The proposed transmission can have an increased number of gear positions without increasing the axial dimension. This transmission will be referred to as a transmission of the three parallel shaft type.
With the transmission of the three parallel shaft type, when changing gear positions, an auxiliary transmission mechanism has to be shifted as well as a main transmission mechanism, resulting in complex transmission control especially for an automatic transmission. Since one more shaft is added, the direction of rotation of the output shaft is opposite to the direction of rotation of the output shaft in the transmission of the two parallel shaft type. Therefore, if the transmission of the three parallel shaft type is employed in place of the transmission of the two parallel shaft type, it is necessary to modify the overall power line of the automobile.